Caption: Charles Darwin in his evolutionary tree. Caricature of the British naturalist Charles Robert Darwin (1809-1882) sitting in his 'evolutionary tree'. Darwin studied the differences in closely related but geographically separated species. The evolutionary tree was the notion that all living things are related and, as different species have evolved from common ancestors, new 'branches' of the tree occur. This notion was first illustrated and popularised in The Origin of Species (1859), which also discussed 'natural selection', the notion that variations in species form arose over time, but only those variations which enhanced a species' chance of survival would be propagated.